Marc Armbrüster

Pd−Ga Intermetallic Compounds as Highly Selective Semihydrogenation Catalysts

The intermetallic compounds Pd(3)Ga(7), PdGa, and Pd(2)Ga are found to be highly selective semihydrogenation catalysts for acetylene outperforming established systems. The stability of the crystal and electronic structure under reaction conditions allows the direct relation of structural and catalytic properties and a knowledge-based development of new intermetallic catalyst systems. In the crystal structure of PdGa palladium is exclusively surrounded by gallium atoms. The alteration of the Pd coordination in PdGa leads to a strong modification of the electronic structure around the Fermi level in comparison to elemental Pd. Electronic modification and isolation of active sites causes the excellent catalytic semihydrogenation properties.

Al13Fe4 as a low-cost alternative for palladium in heterogeneous hydrogenation

Replacing noble metals in heterogeneous catalysts by low-cost substitutes has driven scientific and industrial research for more than 100 years. Cheap and ubiquitous iron is especially desirable, because it does not bear potential health risks like, for example, nickel. To purify the ethylene feed for the production of polyethylene, the semi-hydrogenation of acetylene is applied (80 × 10(6) tons per annum; refs 1-3). The presence of small and separated transition-metal atom ensembles (so-called site-isolation), and the suppression of hydride formation are beneficial for the catalytic performance. Iron catalysts necessitate at least 50 bar and 100 °C for the hydrogenation of unsaturated C-C bonds, showing only limited selectivity towards semi-hydrogenation. Recent innovation in catalytic semi-hydrogenation is based on computational screening of substitutional alloys to identify promising metal combinations using scaling functions and the experimental realization of the site-isolation concept employing structurally well-ordered and in situ stable intermetallic compounds of Ga with Pd (refs 15-19). The stability enables a knowledge-based development by assigning the observed catalytic properties to the crystal and electronic structures of the intermetallic compounds. Following this approach, we identified the low-cost and environmentally benign intermetallic compound Al(13)Fe(4) as an active and selective semi-hydrogenation catalyst. This knowledge-based development might prove applicable to a wide range of heterogeneously catalysed reactions.

A new approach to well-defined, stable and site-isolated catalysts

Abstract A new concept to circumvent some of the problems that are hindering a rational metallic catalyst development is introduced. Investigation of conventional metal catalysts — which consist of supported metals, metal mixtures or alloys — is handicapped by the presence of a variety of active sites, their possible agglomeration, metal–support interactions as well as segregation of the components. In order to avoid most of the drawbacks, we employ well-defined, ordered and in-situ stable unsupported intermetallic compounds. Knowledge of the chemical bonding in the compounds and the defined neighbourhood of the active sites allows a rational approach to catalysts with excellent selectivity as well as long-term stability. The concept is demonstrated for the intermetallic compound PdGa, which is applied as catalyst for the selective hydrogenation of acetylene to ethylene.

How to Control the Selectivity of Palladium-based Catalysts in Hydrogenation Reactions: The Role of Subsurface Chemistry

Discussed are the recent experimental and theoretical results on palladium‐based catalysts for selective hydrogenation of alkynes obtained by a number of collaborating groups in a joint multi‐method and multi‐material approach. The critical modification of catalytically active Pd surfaces by incorporation of foreign species X into the sub‐surface of Pd metal was observed by in situ spectroscopy for X=H, C under hydrogenation conditions. Under certain conditions (low H2 partial pressure) alkyne fragmentation leads to formation of a PdC surface phase in the reactant gas feed. The insertion of C as a modifier species in the sub‐surface increases considerably the selectivity of alkyne semi‐hydrogenation over Pd‐based catalysts through the decoupling of bulk hydrogen from the outmost active surface layer. DFT calculations confirm that PdC hinders the diffusion of hydridic hydrogen. Its formation is dependent on the chemical potential of carbon (reactant partial pressure) and is suppressed when the hydrogen/alkyne pressure ratio is high, which leads to rather unselective hydrogenation over in situ formed bulk PdH. The beneficial effect of the modifier species X on the selectivity, however, is also present in intermetallic compounds with X=Ga. As a great advantage, such PdxGay catalysts show extended stability under in situ conditions. Metallurgical, clean samples were used to determine the intrinsic catalytic properties of PdGa and Pd3Ga7. For high performance catalysts, supported nanostructured intermetallic compounds are more preferable and partial reduction of Ga2O3, upon heating of Pd/Ga2O3 in hydrogen, was shown to lead to formation of PdGa intermetallic compounds at moderate temperatures. In this way, Pd5Ga2 and Pd2Ga are accessible in the form of supported nanoparticles, in thin film models, and realistic powder samples, respectively.

Synthesis and Catalytic Properties of Nanoparticulate Intermetallic Ga–Pd Compounds

A two-step synthesis for the preparation of single-phase and nanoparticulate GaPd and GaPd(2) by coreduction of ionic metal-precursors with LiHBEt(3) in THF without additional stabilizers is described. The coreduction leads initially to the formation of Pd nanoparticles followed by a Pd-mediated reduction of Ga(3+) on their surfaces, requiring an additional annealing step. The majority of the intermetallic particles have diameters of 3 and 7 nm for GaPd and GaPd(2), respectively, and unexpected narrow size distributions as determined by disk centrifuge measurements. The nanoparticles have been characterized by XRD, TEM, and chemical analysis to ensure the formation of the intermetallic compounds. Unsupported nanoparticles possess high catalytic activity while maintaining the excellent selectivity of the ground bulk materials in the semihydrogenation of acetylene. The activity could be further increased by depositing the particles on α-Al(2)O(3).

Nanosizing Intermetallic Compounds Onto Carbon Nanotubes: Active and Selective Hydrogenation Catalysts**

Therefore, nanosizing andsupporting the annealed metal products remain challenges.Another difficulty is in directly preparing supportedcatalysts while simultaneously obtaining good crystallite sizecontrol. A good catalyst support should be capable ofinhibiting sintering and loss of the catalyst during reaction.Fabrication of supported intermetallics catalysts in nanoscaledimensionsrequiresareliablemethodthatfacilitatesnotonlysize control but a thermally stable phase under reactionconditions. Since the work of Iijima in 1991,

Intermetallic compounds in heterogeneous catalysis?a quickly developing field

Abstract The application of intermetallic compounds for understanding in heterogeneous catalysis developed in an excellent way during the last decade. This review provides an overview of concepts and developments revealing the potential of intermetallic compounds in fundamental as well as applied catalysis research. Intermetallic compounds may be considered as platform materials to address current and future catalytic challenges, e.g. in respect to the energy transition.

The Intermetallic Compound ZnPd and its Role in Methanol Steam Reforming

The rich literature about the intermetallic compound ZnPd as well as several ZnPd near-surface intermetallic phases is reviewed. ZnPd is frequently observed in different catalytic reactions triggering this review in order to collect the knowledge about the compound. The review addresses the chemical and physical properties of the compound and relates these comprehensively to the catalytic properties of ZnPd in methanol steam reforming—an interesting reaction to release hydrogen for a future hydrogen-based energy infrastructure from water/methanol mixtures. The broad scope of the review covers experimental work as well as quantum chemical calculations on a variety of Pd-Zn materials, aiming at covering all relevant literature to derive a sound state-of-the-art picture of the understanding gained so far.

High CO2 Selectivity in Methanol Steam Reforming through ZnPd/ZnO Teamwork

Methanol steam reforming (MSR; CH3OH+H2O! 3H2 +CO2) is considered an important building block in the future energy infrastructure to provide clean hydrogen for fuel cell applications. The suppression of CO is the greatest challenge, as the subsequently used fuel cell catalysts only tolerate CO concentrations of up to 50 ppm. Pd/ZnO catalysts have been shown to compete with Cu-based catalysts, and have set benchmarks at about 1000 ppm CO. The formation of the intermetallic compound ZnPd on the ZnO support is held responsible for the high CO2 selectivity. However, no proof of the origin of high CO2 selectivity has been reported thus far. Unraveling the structural properties that account for the selectivity of this catalyst would definitely aid in improving MSR catalysts for practical application. Recently, detailed characterization of the single constituents (unsupported ZnPd and pure ZnO) in MSR resulted in the justified proposal of a bifunctional synergism between intermetallic and oxidic species, which leads to a highly active interface that is necessary for the outstanding selectivity of ZnPd/ZnO catalysts. The bulk composition of ZnPd affects its surface composition and also determines the oxidizability of Zn on the surface. The presence of oxidized Zn in near-surface regions has been shown to be inevitably linked to a high CO2-selectivity in inverse model catalyst studies of near-surface intermetallic phases. The knowledge transfer from these model systems to high-performance catalysts represents a great step towards understanding MSR. Herein we reveal the origin of the high CO2 selectivity of ZnO-supported ZnPd particles in MSR by linking the catalytic properties of ZnPd/ZnO, especially in the initial phase on-stream, with aberration-corrected high-resolution transmission electron microscopy (HRTEM) imaging of the catalyst at different stages of the MSR reaction. ZnPd/ZnO was first examined by TEM and scanning TEM (STEM) after reductive treatment at 773 K (state I). The sample consists of intermetallic particles (ca. 5–50 nm in size) that can be clearly discriminated from the ZnO support (Supporting Information, Figure S3). Figure 1 shows a repre-

Raman effect in icosahedral boron-rich solids

Abstract We present Raman spectra of numerous icosahedral boron-rich solids having the structure of α-rhombohedral, β-rhombohedral, α-tetragonal, β-tetragonal, YB66, orthorhombic or amorphous boron. The spectra were newly measured and, in some cases, compared with reported data and discussed. We emphasize the importance of a high signal-to-noise ratio in the Raman spectra for detecting weak effects evoked by the modification of compounds, accommodation of interstitial atoms and other structural defects. Vibrations of the icosahedra, occurring in all the spectra, are interpreted using the description of modes in α-rhombohedral boron by Beckel et al. The Raman spectrum of boron carbide is largely clarified. Relative intra- and inter-icosahedral bonding forces are estimated for the different structural groups and for vanadium-doped β-rhombohedral boron. The validity of Badgers rule is demonstrated for the force constants of inter-icosahedral B–B bonds, whereas the agreement is less satisfactory for the intra-icosahedral B–B bonds.